Photographic wide angle lens

ABSTRACT

A wide angle lens which is designed to achieve miniaturization without sacrificing optical quality is disclosed.

BACKGROUND OF THE INVENTION

With the development of wide angle lenses, it is natural thatlightweight wide angle lenses would be desirable to permit furtherminiaturization thereof. It is also desirable that such lightweightlenses do not sacrifice efficiency. The wide angle lens according to thepresent invention is lightweight and has good efficiency.

SUMMARY OF THE INVENTION

A retro-focus type wide angle lens consisting of seven lenses which aredivided into a first group and a second group, said first groupconsisting of a first negative meniscus lens convex to the object asecond positive lens, and a third negative meniscus lens convex to theobject said second group comprising a fourth positive lens, a fifthnegative lens, a sixth positive meniscus lens, and a seventh positivelens, said lenses and the spacings therebetween satisfying the followingconditions: ##EQU1##

    (4) 0.5F < r.sub.7 < 0.9F

    (5) 0.4F < d.sub.1 + d.sub.2 + d.sub.3 + d.sub.4 + d.sub.5 + d.sub.6 < 0.8F

    (6) f < r.sub.10 < 2F

    (7) 0.4f < d.sub.7 + d.sub.8 + d.sub.9 < 0.8F,

where;

F is the combined focal length of the seven lenses;

F₁.2..i is the combined focal length of lenses 1 thru i;

n_(i) is the refractive index in d-line of the ith lens;

ν_(i) is Abbe's number for the ith lens;

d_(j) is distance along the optical axis between the j surface and the(j+1) surface of the lenses; and

r_(k) is the radius of curvature of the k surface of the lenses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a lens structure, and

FIG. 2 illustrates curves of aberrations in a preferred embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, there is shown a wide angle lens according to thepresent invention comprising first and second groups of lenses. Thefirst group includes, in combination, a first negative meniscus lens L₁convex to the object a second positive lens L₂, and a third negativemeniscus lens L₃ convex to the object. The second group comprises afourth positive lens L₆, a fifth negative lens L₅, a sixth positivemeniscus lens 6, and a seventh positive lens L₇. The second group isopposed to the first group. The lens satisfies the following conditionsof focal length, radius of curvature, distance and thickness, refractiveindex, and Abbe's number: ##EQU2##

    (4) 0.5F < r.sub.7 < 0.9F

    (5) 0.4F < d.sub.1 + d.sub.2 + d.sub.3 + d.sub.4 + d.sub.5 + d.sub.6 < 0.8F

    (6) f < r.sub.10 < 2F

    (7) 0.4F < d.sub.7 + d.sub.8 + d.sub.9 < 0.8F,

where;

F is the combined focal length of the system;

F₁.2...i is the combined focal length of lenses 1 through i;

n_(i) is the refractive index in d-line of the i lens (as is well knownthe d-line is a special absorption spectral line for Na having a wavelength of about 5890 A -- also known as Fraunhofer's d-line);

ν_(i) is Abbe's number of the i lens;

d_(j) is the thickness or distance between the jth and the j+1 surfacesalong the axis as shown in FIG. 1; and

r_(k) is the radius of curvature of the k surface.

Explanation of Condition (1)

Condition (1) is established to determine the focal length up to thesecond lens, the purpose of which is mainly to correct distortionaberration. It is natural that the condition is connected with F₁, butF₁.2 must be set to maintain a general chromatic aberration in a goodcondition. However, if F₁.2 is longer than F/0.05, the effect ofcorrecting distortion aberration lessens and the purpose of using apositive lens as the second lens tends to vanish. It is further naturalthat where F₁ is extremely negative and has a short focal length, thebalance of correction to F₁.2 is improperly lost. Furthermore, whereF₁.2 is less than F/0.3 and becomes short, there occurs the disadvantagethat it is not only convenient to make the backfocus longer, but alsothe latter group of lenses is quiet forcibly worked in order toaccomplish the object, resulting in a likely loss of balance inaberration.

Explanation of Condition (2)

Condition (2) is established to determine the distribution of thenegative focal length up to the third lens. Where F₁.2.3 is negative andlonger than F/0.9 , it is disadvantageous in terms of backfocus, andwhere other methods are used to compensate for the backfocus, the burdenof the positive lens increases in the second group, and that is notdesirable. This is not proper because it causes trouble in attemptstoward miniaturization. Where F₁.2.3 is less than F/1.2 and becomesshort, it is advantageous for the backfocus but the burden in thenegative degree on the part of r₂, r₆, etc. increases and as a result,coma aberration is relatively high. The provision of the focal length inthe range as described is important to control the amount of sphericalaberration and is also necessary to reduce the number of lenses (thisrelating to Condition (4) later described).

Explanation of Condition (3)

This condition is established to determine the focal length to the firstgroup divided by means of an aperture. With this condition, the focallength of the fifth lens is controlled, and therefore, the condition hasimportant factors related also to the sixth and seventh lenses. WhereF₁.2.3.4 has a focal length longer the F/0.5, it is difficult to correctthe chromatic aberration after the fifth lens and thus the focal lengthto the fifth lens is negative and tends to become short, and as aconsequence, the burden imposed on the sixth and seventh lensesincreases. From this, in correction of aberration, lenses of highrefractive index tend to be used but from a viewpoint of chromaticaberration, it is desirable to use those of small dispersion, with theresult that actually existing potassium nitrate is hard to be selected,resulting in a difficulty of aberration correction. Further, whereF₁.2.3.4 is shorter than F/0.85, the backfocus is likely shortened, andif this is compensated for by shortening the negative focal length ofthe fifth lens, the radius of curvature in opposite surfaces of thefifth lens is unavoidably shortened, resulting in a likely occurrence ofcoma aberration. If this is compensated for by the spacing after thefifth lens, the burden of the sixth and seventh lenses increases, whichis not desirable.

Explanation of Condition (4)

This condition is established to reduce the amount of sphericalaberration in large aperture. This condition is easily influencedparticularly by the condition (2) and d₆. Accordingly, the determinationof the radius of curvature greatly influences the amount of aberration.This is because of excessive exertion of the influence of aberration ina higher order. Where r₇ is shorter than 0.5F, action thereof in ahigher order with respect to the aberration becomes too great so thateven if suitable correction should be made in a region of a zonal ray,the marginal ray would create the spherical aberration in a direction ofexcessively under correction. Conversely, where r₇ is greater than 0.9F,action is too short so that over correction excessively remains.

Explanation of Condition (5)

Although intimately related to the conditions (2) and (4) as previouslydescribed, this condition (5) is established for the purpose ofdetermining the range of miniaturization and of selecting the range soas not to worsen the aberration. This condition is also related to thethickness of lens and spacing, but it is natural that d₆ must have asuitable value in order to maintain the backfocus for a long period oftime as necessary. It is preferred, however, that the value in variationof d₆ and the value in variation of the sum from d₁ to d₅ are summed upand the quantity of light may suitably be adjusted under the condition(5). Where the value of d₁ + d₂ + d₃ + d₄ + d₅ + d₆ is smallar than0.4F, the object of miniaturization may be attained but a decrease inthe quantity of light or an aggravation in coma aberration with respectto a beam of light lower in incidence tends to occur, which is notdesirable. Where the aforesaid value is greater than 0.8F, it may beadvantageous in terms of aberration correction, but opposed to theobject of miniaturization since the front lens will likely increase itsdiameter.

Explanation of Condition (6)

This condition is established to prevent coma aberration from beingworsened. In the present optical system, it is advantageous to employthe fifth lens, which is small in the ν value, relative to the chromaticaberration correction in the latter group, and hence, the necessity ofusing potassium nitrate in refractive index occurs. For this reason, thecondition (6) is required since the refractive power here tends toresult in the disturbance of the image in the form of a beam of lightrelatively lower in incidence. That is, where r₁₀ is smaller than F, theabove-described beam of light likely becomes an outward coma, whereaswhere it is longer than 2F, the beam of light likely becomes an inwardcoma.

Explanation of Condition (7)

This condition has an intimate relation with the condition (6) and isestablished to properly maintain the balance between the seventh surfaceand the tenth surface and is also related to the object of achievingminiaturization. Where the value of d₇ + d₈ + d₉ is greater than 0.8F,and if the fourth lens and the fifth lens (including r₁₀) sufficientlyperform their function so as to maintain the balance of aberration, boththe lenses in the former and latter groups are unavoidably designedgreater in size, which is opposed to the object of miniaturization.Where the aforesaid value is smaller than 0.4F, the action of the fourthand fifth lenses becomes dull, and particularly, action of the seventhsurface and the tenth surface is not progressed as desired to therebylose the balance of aberration. Particularly in the case of brightnessat the ratio of about 1:2, this effect is notable.

One example is as shown in the following:

EXAMPLE

F = 100

f₁.2 = 653.59

f₁.2.3 = -96.15

f₁.2.3.4 = 141.84

for reference: The sum of Petzval: 0.198

    __________________________________________________________________________         Radius of                                                                              Lens Thickness                                                                        Refractive                                                                           Abbe                                             Lens Curvature r                                                                            or Distance d                                                                         Index n                                                                              No. ν                                         __________________________________________________________________________         r.sub.1 =                                                                         161.15                                                               L.sub.1                                                                                     d.sub.1 =                                                                         6.15                                                                              n.sub.1 = 1.60738                                                                    ν.sub.1 = 56.8                                     r.sub.2 =                                                                         72.55                                                                              d.sub.2 =                                                                         19.93                                                            r.sub.3 =                                                                         186.64                                                               L.sub.2                                                                                     d.sub.3 =                                                                         10.50                                                                             n.sub.2 = 1.71300                                                                    ν.sub.2 = 53.9                                     r.sub.4 =                                                                         -402.24                                                                            d.sub.4 =                                                                         0.59                                                             r.sub.5 =                                                                         1834.76                                                              L.sub.3                                                                                     d.sub.5 =                                                                         6.41                                                                              n.sub.3 = 1.56873                                                                    ν.sub.3 = 63.1                                     r.sub.6 =                                                                         49.54                                                                              d.sub.6 =                                                                         14.12                                                            r.sub.7 =                                                                         65.07                                                                L.sub.4                                                                                     d.sub.7 =                                                                         30.23                                                                             n.sub.4 = 1.79952                                                                    ν .sub.4 = 42.2                                    r.sub.8 =                                                                         -362.02                                                                            d.sub.8 =                                                                         10.91                                                            r.sub.9 =                                                                         -69.86                                                               L.sub.5                                                                                     d.sub.9 =                                                                         7.23                                                                              n.sub.5 = 1.80518                                                                    ν.sub.5 = 25.4                                     r.sub.10 =                                                                        152.00                                                                             d.sub.10 =                                                                        6.69                                                             r.sub.11 =                                                                        -235.10                                                              L.sub.6                                                                                     d.sub.11 =                                                                        11.40                                                                             n.sub.6 = 1.73400                                                                    ν.sub.6 = 51.5                                     r.sub.12 =                                                                        -60.16                                                                             d.sub.12 =                                                                        2.00                                                             r.sub.13 =                                                                        381.07                                                               L.sub.7                                                                                     d.sub.13 =                                                                        12.45                                                                             n.sub.7 = 1.64000                                                                    ν.sub.7 = 60.2                                     r.sub.14 =                                                                        -105.28                                                              __________________________________________________________________________     All linear dimensions given above are in millimeters.                    

What is claimed is:
 1. A retro-focus type wide angle lens systemcomposed of seven lenses in two groups, the first group consisting of afirst negative meniscus lens L₁ convex to the object, a second positivelens L₂, and a third negative meniscus lens L₃ convex to the object, andthe second group consisting of a fourth positive lens L₄, a fifthnegative lens L₅, a sixth positive meniscus lens L₆, and a seventhpositive lens L₇, wherein the focal lengths, the radii of curvature r₁to r₁₄, the lens thickness or distance d₁ to d₁₃, the refractive indicesn₁ to n₇, and the Abbe numbers ν₁ to ν₇ for the lenses L₁ to L₇ satisfythe following conditions:F = 100, where F is the focal length of thewhole system, F₁.2 =653.59, where F₁.2 is the combined focal length oflenses L₁ and L₂, f₁.2.3 = -96.15, where F₁.2.3 is the combined focallength of lenses L₁ through L₃, and F₁.2.3.4 = 141.84, where F₁.2.3.4 isthe combined focal length of lenses L₁ -L₄

    ______________________________________                                                          Lens                                                              Radius of   Thickness  Refractive                                                                             Abbe                                    Lens  Curvature r or Distance d                                                                            Index n  No. ν                                ______________________________________                                                  r.sub.1 =                                                                            161.15                                                       L.sub.1                                                                                               d.sub.1 =  6.15                                                                        n.sub.1 = 1.60738                                                                      ν.sub.1 = 56.8                             r.sub.2 =                                                                            72.55                                                                                d.sub.2 =  19.93                                                r.sub.3 =                                                                            186.64                                                       L.sub.2                                                                                               d.sub.3 =  10.50                                                                       n.sub.2 = 1.71300                                                                      ν.sub.2 = 53.9                             r.sub.4 =                                                                            -402.24                                                                              d.sub.4 =  0.59                                                 r.sub.5 =                                                                            1834.76                                                      L.sub.3                                                                                               d.sub.5 =  6.41                                                                        n.sub.3 = 1.56873                                                                      ν.sub.3 = 63.1                             r.sub.6 =                                                                            49.54                                                                                d.sub.6 =  14.12                                                r.sub.7 =                                                                            65.07                                                        L.sub.4                                                                                               d.sub.7 =  30.23                                                                       n.sub.4 = 1.79952                                                                      ν.sub.4 = 42.2                             r.sub.8 =                                                                            -362.02                                                                              d.sub.8 =  10.91                                                r.sub.9 =                                                                            -69.86                                                       L.sub.5                                                                                               d.sub.9 =  7.23                                                                        n.sub.5 = 1.80518                                                                      ν.sub.5 = 25.4                             r.sub.10 =                                                                           152.00                                                                               d.sub.10 =  6.69                                                r.sub.11 =                                                                           -235.10                                                      L.sub.6                                                                                               d.sub.11 =  11.40                                                                      n.sub.6 = 1.73400                                                                      ν.sub.6 = 51.5                             r.sub.12 =                                                                           -60.16                                                                               d.sub.12 =  2.00                                                r.sub.13 =                                                                           381.07                                                       L.sub.7                                                                                               d.sub.13 =  12.45                                                                      n.sub.7 = 1.64000                                                                      ν.sub.7 = 60.2                             r.sub.14 =                                                                           -105.28                                                      ______________________________________                                    